Assembling a stacked die package

ABSTRACT

A technique for assembling a multi-chip module does not necessitate more than one pass through a die attach machine. The technique involves attaching a smaller die to a larger die without using a die attach machine. The larger die may be attached to a support structure using a die attach machine. In some embodiments, the larger die is affixed to the support structure first and in other embodiments, the smaller die is affixed to the larger die first.

BACKGROUND

[0001] This invention relates generally to assembling integrated circuitdice into packages.

[0002] For a number of reasons, it may be desirable to package two dicetogether in a single package. For example, the components provided in aseparate die may not be capable of being integrated in a singleintegrated circuit. For example, different process technologies may beused in each die. In addition, basic incompatibilities betweencomponents on each die may necessitate separate die fabrication. Oncethe dice are fabricated independently, it may be desirable to packagethem in a single package so that a single set of contacts may be used toconnect with each of the dice.

[0003] The handling of dice, which may be stacked, to form a compositecreates a number of difficulties. Predominantly, semiconductor assemblyequipment is adapted to produce packages containing a single die. Thus,techniques are needed to assemble the dice.

[0004] Conventional dice assembly techniques may involve a pair ofpasses through a die attach machine. For example, a larger of two diceto be stacked may be attached to a support structure in a die attachmachine. Thereafter, the support structure may be run through the dieattach machine again to attach the smaller die to a larger die.

[0005] The repeated passes through the die attach machine may bedisadvantageous for a number of different reasons. For example, theassembly process is complicated by the multiple passes through the dieattach machine. In some cases the die attach process may be relativelyexpensive. In some cases the thermal budget may be impacted by the dieattach process.

[0006] Die attach machines usually use an organic-based adhesive toattach the support structure to the die or to attach one die to anotherdie. The adhesive may be applied in an uncured or soft phase between thesurfaces to be joined. The adhesive is then cured through exposure toelevated temperatures for a specified time period. The carrier materialis typically an epoxy resin or polyimide. The carrier provides theadhesion and mechanical strength along the bond line. The carrier may befilled with metal particles in the event electrical and thermalconductivity is required or with non-metallic particles such as alumina,if the ultimate bond should be an insulator.

[0007] Three types of polymer adhesives that are often used in dieattach processes are epoxies, cyanate esters and polyimides. Epoxies userelatively low cure temperatures (125-175° C.) and have moderate glasstransition temperatures (100-155° C.). However, epoxies vary in theirability to absorb and outgas moisture and other contaminants. Cyanateester adhesives use higher cure temperatures (300° C.) and haverelatively higher glass transition temperatures (240° C.). However,unlike epoxy adhesives, cyanate esters have low outgassingcharacteristics as well as low ionic contents. Finally, polyimideadhesives use high cure temperatures (greater than 400° C.) and likecyanate esters have a high glass transition temperature. A variety ofconductive fillers including silver, gold or copper and non-conductivefillers, such as alumina, diamond, glass fabric, silica or ceramic maybe used with the adhesive.

[0008] Thus, in addition the complexity inherent in the die attachprocess, in some cases the thermal exposure may be disadvantageous bothto shallow junctions in advanced devices and to the wire bonds or otherelements whose electrical performance may be affected.

[0009] Circuits that are susceptible to outgassing and moisturegenerally use a metallurgical attachment technique. Solder or metaleutectic alloys may provide good thermal conductivity but because oftheir electrical conductivity they may not be used if the backside ofthe die must be insulated from the support structure. Solders andeutectic alloys generally use higher processing temperatures and therebyimpose constraints on work flow. For example, if a gold-tin die attachoperation is scheduled after other chips in a multi-chip module havebeen wire bonded, the high temperatures used may degrade the wire bondsor degrade the electrical performance of the actual device.

[0010] Thus, for a variety of reasons, it may be desirable in makingmulti-chip modules, to reduce the number of times that the module or anypart thereof must be exposed to the die attach process.

SUMMARY

[0011] In accordance with one aspect, a method of assembling multi-chipmodules includes using a die attach machine to attach a first,relatively larger die to a support structure. A second, relativelysmaller die is affixed to the larger die without using a die attachmachine.

[0012] Other aspects are set forth in the accompanying detaileddescription and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a process flow in accordance with one embodiment of thepresent invention;

[0014]FIG. 2 is a more detailed embodiment of a process flow shown inFIG. 1;

[0015]FIG. 3 is a greatly enlarged top plan view of a die positioned ona wafer;

[0016]FIG. 4 is a greatly enlarged cross-sectional view of a smaller diepositioned on a larger die;

[0017]FIG. 5 is a greatly enlarged cross-sectional view of a multi-chipmodule;

[0018]FIG. 6 is a more detailed process flow for a process in accordancewith the more general flow show in FIG. 1;

[0019]FIG. 7 is a top plan view of a die positioned on a wafer tape;

[0020]FIG. 8 is a top plan view of a smaller die positioned on a largerdie positioned on a wafer tape;

[0021]FIG. 9 is a more detailed flow for another embodimentcorresponding to the more general flow shown in FIG. 1;

[0022]FIG. 10 is a top plan view of a larger and a smaller die inposition on a wafer tape;

[0023]FIG. 11 is an enlarged, cross-sectional view showing a larger diethat has been die attached to a support structure; and

[0024]FIG. 12 is a greatly enlarged cross-sectional view of a multi-chipmodule.

DETAILED DESCRIPTION

[0025] A process for forming a multi-chip module, shown in FIG. 1,avoids the need to expose the elements of the module to a die attachmachine more than one time. As used herein, a multi-chip module isintended to refer to a combination of two dice stacked one on top of theother, and coupled to a support structure. A support structure may beany one of a variety of devices used in making multi-chip modulesincluding leadframes, substrates, and laminate cores, as examples.

[0026] The process flow may include three basic steps. One step is todie attach the larger die to the support structure (block 10). This stepis conventionally done in a die attach machine. Another step is to affixa smaller die to the larger die without using a die attach machine, asillustrated by block 12. Finally, the composite of the smaller die overthe larger die over a support structure may be packaged as indicated inblock 14.

[0027] Two different sequences may be utilized to assemble themulti-chip module. In accordance with one sequence, the smaller die maybe affixed to the larger die and then that subassembly may be dieattached to a support structure as indicated by the arrows A1 in FIG. 1.Thereafter, the assembly may be packaged as indicated by arrows B1.

[0028] In accordance with another sequence, initially, the larger diemay be die attached to the support structure. Thereafter, as indicatedby arrow A2, the smaller die may be affixed to the larger die which hasalready been affixed to the support structure. Finally, as indicated byarrow B2, the assembly may be packaged.

[0029] In general, it may be advantageous, in some embodiments, if thesmaller die is sufficiently smaller than the larger die to enable bothdice to be wire bonded after the smaller die is attached to the largerdie, with both dice facing upwardly.

[0030] A more detailed process, shown in FIG. 2, for implementing one ofthe sequences illustrated in FIG. 1, may begin by singulating a smallerdie from a wafer as indicated in block 16. The smaller die may then beattached to the larger die as indicated in block 18. In accordance withone aspect of the invention, this attachment is done without using a dieattach machine. Referring to FIG. 3, a semiconductor wafer 26 is shownwith a smaller die 30 attached to an unsingulated, larger die 28 stillin wafer form.

[0031] The smaller die 30 may be attached to the larger die 28 withoutusing a die attach machine, but using conventional securementtechniques. For example, adhesive tape such as lead on chip (LOC) tape,adhesives that do not need to be cured at temperatures above roomtemperature, solder or other non-die attach machine based techniques maybe used.

[0032] For example, double sided die attach tape may be situated betweenthe smaller and larger dice. A suitable tape is Sumioxy 5120T adhesive,75 to 125 micrometers thick. Bond pads for coupling the external devicesmay be exposed peripherally about the larger die 28 so as to beaccessible around the smaller die 30. A pick and place machine may beused to locate the smaller die over the larger die.

[0033] Thereafter, the larger die 28 is singulated as indicated in block20 of FIG. 2. FIG. 4 shows a cross-sectional view of the smaller die 30attached by a securement medium 33 to the larger die 28 after the largerdie has been singulated in accordance with the step illustrated by block20.

[0034] The stack of the smaller die 30 and the larger die 28 may then bedie attached to a support structure, as indicated in block 22 in FIG. 2.Similarly, FIG. 5 shows the subassembly of the smaller die 30 and largerdie 28 attached to a support structure 34 through a die attach 32. Thedie attach 32 may be a heat curable adhesive deposited bysyringe-dispensing, screen printing or stamping paste adhesive, asexamples. Alternatively, perforated film adhesives may be used as thedie attach. Thereafter, conventional packaging techniques may beutilized, as indicated in block 24 in FIG. 2, including encapsulation.

[0035] The die attach process may be otherwise conventional in all ways.Thus, depending on the adhesive that is utilized, the necessary time andtemperature may be determined, as will be apparent to those skilled inthe art. For example, a conductive adhesive may be formed as an epoxypaste with a silver filler. Its glass transition temperature is 127° C.Similarly, a nonconductive adhesive may be formed of epoxy paste with analumina filler. Its glass transition temperature is 85° C.

[0036] In accordance with still another embodiment, a flow forassembling a multi-chip module using only one pass through a die attachmachine may begin by locating a larger die on a wafer tape asillustrated at block 40 in FIG. 6. The wafer tape, sometimes called awafer dicing tape, is a tape with high toughness used to hold wafersduring the dicing process. An example of wafer tape is a polyvinylchloride membrane with adhesive applied to one side and mounted in aframe.

[0037] Referring to FIG. 7, the larger die 28 is shown in position onthe adhesive side of a wafer tape 48. The tape 48 includes an adhesivecovered polymer membrane 52 secured in a frame 50. The frame 50 isillustrated by a rectangular frame but other shapes includingring-shaped frames may be utilized as well.

[0038] The die 28 is held in position by the adhesive on the membrane52. While only a single die is shown in position on the wafer tape 48 inFIG. 7, one or more dice may be positioned on the tape 48 at one time,in some embodiments of the present invention.

[0039] Referring back to FIG. 6, the next step is to attach the smallerdie to the larger die using a pick and place machine. A pick and placemachine transfers the die 28 from the wafer tape 48 onto the larger die28, as indicated in block 42. A die attach material may be positionedbetween the stacked die. The result, shown in FIG. 8, has a smaller die30 on top of the larger die 28 still secured to the wafer tape 48.

[0040] The stacked dice are then die attached to a support structure asindicated in block 44. In the die attach machine, the die attach layersbetween the dice and between the layer die and the support structure maybe simultaneously activated and cured. The resulting composite, shown inFIG. 12, may use die attach material as the layers 32. Thereafter, thecomposite is packaged as indicated in block 46 in FIG. 6.

[0041] In accordance with still another embodiment, a larger die 28 maybe located onto the wafer tape 48 as indicated in block 54 in FIG. 9.Thereafter, a smaller die 30 may be attached next to the larger die 28on the wafer tape 48 as indicated in block 56. Thus, referring to FIG.10, the larger die 28 may be positioned next to the smaller die 30 onthe adhesive bearing side of the wafer tape 48.

[0042] The larger die 28 then may be stacked on a support structure, asindicated in block 58 in FIG. 9, using a die attach machine. Theresulting composite is shown in FIG. 11. The larger die 28 is mounted ona die attach 32 over the support structure 34.

[0043] Next the smaller die 30 is mounted onto the larger die 28 (block60 in FIG. 9). This may be done using the die attach machine. Thecomposite of the die attach materials, the larger and smaller dice 28and 30 and the support structure 34 is then secured together in a dieattach machine. The composite, shown in FIG. 12 includes die attachlayers 32 between the dice 28 and 30 and the support structure 34. Thecomposite is then packaged as indicated in block 62 in FIG. 9.

[0044] While the present invention has been described with respect to alimited number of embodiments, those skilled in the art will appreciatenumerous modifications and variations therefrom. It is intended that theappended claims cover all such modifications and variations as fallwithin the true spirit and scope of this present invention.

What is claimed is:
 1. A method of assembling multi-chip modulescomprising: using a die attach machine to attach a first, relativelylarger die to a support structure; and affixing a second, relativelysmaller die to a larger die without using a die attach machine.
 2. Themethod of claim 1 wherein using a die attach machine occurs beforeaffixing a smaller die to a larger die.
 3. The method of claim 1 whereinaffixing a smaller die occurs before using a die attach machine toattach a larger die to a support structure.
 4. The method of claim 1wherein said smaller die is mounted on said larger die using a pick andplace machine.
 5. The method of claim 1 wherein said smaller die ismounted on said larger die when said larger die is still part of awafer.
 6. The method of claim 1 including attaching a larger die to awafer tape and then mounting the smaller die on a larger die.
 7. Themethod of claim 6 including mounting the smaller die to larger die stillon the wafer tape.
 8. The method of claim 7 including positioning thesmaller die and the larger die next to one another on the wafer tape. 9.The method of claim 8 including attaching a larger die to a supportstructure and then attaching the smaller die to the larger die.
 10. Themethod of claim 9 including using a pick and place machine to pick upthe larger die and attach it to a support structure and to pick up thesmaller die and attach it on top of the larger die.
 11. A method ofassembling multi-chip modules comprising: singulating a first,relatively smaller die; attaching the smaller die to a second,relatively larger die; singulating the larger die; and attaching thesmaller and larger dice to a support structure.
 12. The method of claim11 including using a die attach machine to attach said smaller andlarger dice to said support structure.
 13. The method of claim 11including using a pick and place machine to mount said smaller die tosaid larger die.
 14. A method of assembling multi-chip modulescomprising: locating a first, relatively larger die on a wafer tape;attaching a second, relatively smaller die to a larger die; and dieattaching the attached dice to a support structure.
 15. The method ofclaim 14 including using a pick and place machine to mount said smallerdie on said larger die.
 16. The method of claim 14 including adhesivelysecuring said larger die to said wafer tape.
 17. A method of assemblingmulti-chip modules comprising: locating a larger die on a wafer tape;attaching a smaller die next to a larger die on said wafer tape;mounting a larger die to a support structure; mounting said smaller dieon said larger die; and securing said smaller die and said larger die onsaid support structure.
 18. The method of claim 17 including using apick and place machine to locate said larger die on said wafer tape. 19.The method of claim 18 including adhesively securing said larger andsmaller dice to said wafer tape.
 20. The method of claim 19 includinglocating a die attach material between said larger and smaller dice. 21.The method of claim 20 including locating a die attach material betweensaid larger die and said support structure.
 22. The method of claim 21including curing said die attach material between said dice and betweensaid support structure and said larger die at one time in a die attachmachine.
 23. A method of making a multi-chip module comprising:positioning a first, relatively smaller die on a second, relativelylarger die; positioning the larger die on a support structure; andsecuring the smaller die to the larger die and securing said larger dieto a support structure by way of a single pass through a die attachmachine.
 24. The method of claim 23 including using a die attach machineoccurs before positioning the smaller die on the larger die.
 25. Themethod of claim 23 including affixing a smaller die on the larger dieand then using a die attach machine to attach a larger die to a supportstructure.
 26. The method of claim 23 including positioning said smallerdie on said larger die using a pick and place machine.
 27. The method ofclaim 23 including positioning said smaller die on said larger die whensaid larger die is still part of a wafer.
 28. The method of claim 23including attaching a larger die to a wafer tape and then positioningthe smaller die on a larger die.
 29. The method of claim 28 includingpositioning the smaller die on the larger die still on the wafer tape.30. The method of claim 29 including positioning the smaller die and thelarger die next to one another on the wafer tape.
 31. The method ofclaim 30 including attaching a larger die to a support structure andthen attaching the smaller die to the larger die.
 32. The method ofclaim 31 including using a pick and place machine to pick up the largerdie and attach it to a support structure and to pick up the smaller dieand attach it on the larger die.